Use runtime::FunctionRef in threadpool.h itself

[swolchok]

I previously added runtime::FunctionRef and used it in parallel_for to save code size, but didn't carry that through to the Threadpool wrapper. Profiling showed that we were doing heap allocations on each parallel_for invocation, so let's fix that.

[swolchok]

Stack from ghstack (oldest at bottom):

• -> Use runtime::FunctionRef in threadpool.h itself #15265

[pytorch-bot]

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❌ 2 New Failures, 4 Unrelated Failures

As of commit c40189a with merge base 2e8e579 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t 79f058dce9d4dcfc14253632c438ccbb452a35fa2448a20fe81fcfd5ab98a083 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 4ee76b519ee5296741b7489c07165662e60b3f9d92d50bcc92d8eebfe7b48b04 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / unittest / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest-editable / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / android / run-emulator (gh) (trunk failure)
  The process '/usr/bin/sh' failed with exit code 255
• pull / unittest / linux / linux-job (gh) (trunk failure)

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